數學題目疑問

You may consider the following method:
(1) Forget about the y in the expression, we get x^2 + 6x + 9 = (x + 3)(x + 3).
(2) Add back the y we get the answer ( x + 3y)(x + 3y) = (x + 3y)^2.

x^2+6xy+9y^2=x^2 + 2(x)(3y) + (3y)^2
=(x+3y)^2

a^2 + 2ab + b^2 = (a+b)^2

所以應該係
(x+3y)(x+3y)